Electronic labels are devices which are able to be attached to various objects so that information relating to these objects, stored on the electronic label, may be easily obtained. Therefore, one may affix labels to objects such as assets, animals, and people so that the information may be obtained using a wireless device such as an interrogator.
An electronic labelling system for managing objects having electronically coded labels may be of a type in which information passes between an interrogator, which creates an electromagnetic interrogation field, and electronically coded labels (‘the labels’) which are able to respond by issuing a reply signal which is able to be detected by the interrogator and possibly supplied to other apparatus.
In normal operation the labels may be passive (that is, the labels have no internal energy source and obtain energy for their reply from the interrogation field), or active (that is, the labels contain an internal energy source, for example a battery) and respond only when they are within, or have recently, passed through the interrogation field which may have the function of signalling to an active label when to commence a reply or a series of replies.
It appears that a common problem in existing electronic label systems is that when an unknown plurality of labels are simultaneously present in an interrogation field, a communication process between the interrogator and label may be required to be structured so that all of the labels present in the interrogation field are able to be detected, often within a short time.
A protocol for communicating with an unknown plurality of labels simultaneously present in an interrogation field may be of the type described in patent application PCT AU 92 00143 (the '143 application). The '143 application describes a protocol in which labels may repeat their replies intermittently and at varying intervals with spaces between the replies significantly greater than the duration of a reply, so that over time all replies may be detected without interference. Whilst the protocol described in the '143 application has an advantage of requiring little or no signalling for the interrogator, the protocol exhibits poor performance when there is a large number of labels simultaneously in the interrogation field.
In other multiple label read protocols, significant signalling from the interrogator to each label is often required. Here, because of a need to keep label circuits simple, interrogator signalling may be performed by amplitude modulating an interrogator powering and carrier signal (‘the interrogator signal’) to provide ‘dips’ in interrogation carrier power. In this respect, reference to the term ‘dips’ throughout this specification is to be understood to be reference to a localised minima in the interrogator signal amplitude which have been produced by amplitude modulating the interrogator powering and carrier signal using a modulating signal which includes command or data content.
When the interrogator powering and carrier signal frequency is too high to provide a convenient time reference in a label circuit, an oscillator (herein referred to as an ‘on-chip oscillator’) included in the label, may provide the timing reference against which details of interrogator signal dips are able to be examined to extract command or data content. However, due to manufacturing tolerances and/or substantial variation in excitation levels experienced by labels as they pass through the interrogation field at various orientations, on-chip oscillators may suffer from poor frequency stability. Such poor frequency stability may make the label susceptible to extracting incorrect command or data information from the interrogator signal.
Existing electronic labelling systems attempt to solve this problem by beginning the interrogator signalling with a long period fixed frequency amplitude modulation signal for training the on-chip oscillator to a standard frequency. However, time taken to train the on-chip oscillator inhibits rapid execution of multiple label reading algorithms. Moreover, such a solution imposes a significant burden on attempts satisfy electromagnetic compatibility regulations, which generally do not permit much signalling by the interrogator.
In relation to approaches adopted by existing electronic labelling systems to satisfy the electromagnetic compatibility regulations, existing systems utilise interrogator amplitude modulation pulses having a substantial width, so that shaping to allow out of band sideband reduction can be employed. In such approaches, the dips may take the form of raised cosine pulses on substantial base lines, or long rise and fall times may be applied to previously rectangular pulses, or various forms of low pass filtering may be applied to a modulation envelope.
These solutions have, however, the substantial disadvantage of requiring a significant size reservoir capacitor in a rectifier circuit included in the label, so that label circuits can be sustained with operating power during the period of the wide interrogator signal dips.
In light of the preceding discussion it can therefore be appreciated that there appear to be numerous problems associated with electronic labelling systems. It is thus an aim of the present invention to ameliorate these deficiencies.